Overview

A Bose–Einstein condensate (BEC) is a state of matter that emerges when a dilute collection of particles behaves as a single quantum entity. It typically forms in an ultracold, dilute gas of atoms cooled toward absolute zero (0 K, −273.15 °C, −459.67 °F). At such low temperatures the thermal motion becomes negligible and a large fraction of the particles occupy the lowest available quantum state, producing long-range coherence and collective behaviour that can be observed on macroscopic scales.

Formation and theory

Only particles with integer spin, called bosons, can occupy the same single-particle quantum state and form a BEC directly. The transition to a condensate is a type of quantum phase transition driven by temperature and particle density: when the thermal de Broglie wavelengths of particles become comparable to the spacing between them, individual wavefunctions overlap and a macroscopic occupation of the ground state appears. This requires very low energy and results in a system described by a single coherent matter wave. Concepts used to analyze BECs include the condensate fraction, coherence length and collective excitation modes.

Experimental realization

The possibility of Bose–Einstein condensation was predicted by Satyendra Nath Bose and Albert Einstein in the 1920s. Laboratory BECs were first achieved in 1995 using rubidium atoms by teams led by Eric Cornell and Carl Wieman, with later important contributions from Wolfgang Ketterle; these advances were recognized with the 2001 Nobel Prize in Physics. Creating a BEC typically involves successive cooling and trapping techniques such as laser cooling, magneto-optical traps, magnetic or optical dipole traps and evaporative cooling to reach quantum degeneracy.

Properties

Condensates exhibit macroscopic quantum phenomena. When a large fraction of particles occupies the same quantum state, the system shows high coherence and can display superfluidity, quantized vortices, collective oscillations and very low effective viscosity. The phrase "zero viscosity" is used descriptively for some superfluid behaviours, but experimental properties depend on interactions, confinement and temperature. Atomic BECs are usually extremely dilute compared with liquids like helium and are therefore well described by mean-field theories in many regimes.

  • BEC vs superfluid helium: superfluid helium is a dense, strongly interacting liquid, while atomic BECs are dilute gases with weak interactions; both show superfluid phenomena but differ in microscopic details.
  • Fermions and pairing: fermions cannot occupy the same single-particle state, but paired fermions (Cooper pairs) or bound molecules of fermionic atoms can behave like bosons and condense, leading to the BEC–BCS crossover explored in ultracold gases.
  • Matter-wave interference: macroscopic coherence permits direct observation of interference between condensates, demonstrating their wave-like nature.

Applications and research uses

BECs are a versatile platform for studying quantum mechanics at accessible length and time scales. They are used to investigate fundamental many-body physics, quantum vortices and solitons, and non-equilibrium dynamics. Practical applications and experimental techniques based on condensates include atom interferometry for precision sensing, quantum simulation of condensed-matter models and development of coherent atomic sources sometimes called "atom lasers." Ongoing research explores tunable interactions, low-dimensional condensates and mixtures of different atomic species.

Further reading